Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA) based on European systems, Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS). Since the 1999 release of the basic 3GPP specifications for WCDMA, there have been several releases which improve on various aspects of that 1999 release. This continuing improvement is sometimes referred to as Long Term Evolution (LTE). In release 5 of the WCDMA 3GPP specifications, high speed downlink packet access (HSPDA) was introduced to reduce downlink delays and increase downlink data rate capability by approximately a factor of three. Release 6 of the WCDMA 3GPP specifications also reduces uplink delays and increases uplink data rate capability by approximately a factor of two.
Release 6 introduces a new uplink transport channel called the Enhanced Dedicated Channel (E-DCH) targeted for interactive, background, and streaming traffic. Compared to the normal uplink DCH, the E-DCH achieves improved uplink performance using a short transmission time interval (TTI), hybrid ARQ with soft combining, and scheduling. Reducing the TTI allows for an overall reduction in delay and faster hybrid ARQ retransmissions. Fast hybrid ARQ with soft combining reduces the number of retransmissions as well as the time between retransmissions. It also allows for a significant increase in capacity. Fast scheduling allows for rapid resource reallocation between UEs, exploiting the burstiness in packet data transmissions. It also allows the system to admit a larger number of high data rate users and rapidly adapt to interference variations, thereby leading to an increase in capacity as well as an increase in the likelihood that a user will experience high data rates. The functionality for controlling retransmission delay for hybrid ARQ and fast scheduling is implemented in the base station sometimes called the Node B in UMTS parlance.
In the downlink HSPDA, the transmission power and the code space is the shared resource, but in the uplink E-DCH, the interference “headroom” is the amount of shared resource (i.e., transmit power or interference) left to be allocated to one or more mobile terminals to transmit in the uplink. The common uplink resource shared among the mobile radio terminals (sometimes called user equipment (UE) in UMTS) is the total amount of tolerable interference, i.e., the total received power at the base station. The amount of common uplink resources allocated to a mobile terminal depends on the data rate (transport format) to be used. Generally, the higher the data rate, the larger the required transmission power/interference, and thus, the higher the resource consumption.
Scheduling is the mechanism that determines when a certain mobile terminal is allowed to transmit and at what maximum data rate. Packet data applications are typically bursty in nature with large and rapid variations in their resource requirements. Hence, the overall goal of the uplink scheduler is to allocate a large fraction of the shared resource to users momentarily requiring high data rates, while at the same time ensuring stable system operation by avoiding sudden interference peaks. Identifying this goal is one thing; achieving it is another.
The uplink dedicated channels DCHs in WCDMA are “fast” power-controlled, meaning that the base station measures the received DPCCH signal quality, e.g., the received signal to interference ratio (SIR), and compares the measurement to a desired signal quality, e.g., a SIR target value. If the measured SIR is less than or equal to the SIR target, the base station signals an “up” power control command to the mobile terminal to make it increase the power by a predefined step and a “down” power control command to the mobile terminal to make it increase its power by a predefined step if the received SIR is greater than the SIR target. The SIR target is regularly updated in a “slow” power control procedure known as outer loop power control (OLPC).
Because fast power control is used for the uplink, a mobile terminal transmitting when the channel conditions are favorable will generate the same amount of interference in the cell as a terminal transmitting in unfavorable channel conditions, given the same data rate for the two and sufficient power in the terminals to obey the power control commands. This is in contrast to HSDPA, where principally a constant transmission power is used, and the data rates are adapted to the channel conditions, resulting in a higher data rate for users with favorable radio conditions. However, for the uplink case the transmission power will be different for the two mobile terminals, and hence, the amount of interference generated in neighboring cells will differ.
The uplink scheduler for the enhanced uplink channel must be provided with an estimate of the allowed headroom for a cell for each scheduling period so that the scheduler can allocate the appropriate amount of uplink resources to requesting mobile terminals. But granting too much power to requesting mobile terminals can result in unstable power control conditions such as the “party effect” where an active mobile terminal responds to other terminals having increased their transmit power by increasing its own transmit power (the analogy being to a party where people all talk louder in an attempt to be heard in a louder environment). The effect is a power rush where multiple mobile terminals compete for signal to noise ratio (SIR) by raising their transmit power in an attempt to maintain their respective signal quality in an interference limited situation. A power rush can also occur when a terminal aims to achieve a higher SIR than what is possible due to the own-signal interference caused by time dispersion. When a mobile terminal's SIR reaches that region, the inner loop fast power control will not be able to reach the target SIR resulting in the base station sending a constant stream of power-up commands to the mobile terminal with a high risk of a power rush as the mobile terminal follows those commands.
In theory, a power rush may be depicted as a situation where the interference level goes to infinity, and the cell is completely blocked by the interference. In reality, the mobile terminals have limited output power which means that the interference in the cell will be high and many mobiles may lose link coverage leading to increased drop rate, increased inter-cell interference in neighboring cells, and reduced system capacity. Moreover, the terminals will use unnecessarily high power resulting in reduced battery life time. It would be desirable to avoid these unstable situations and to utilize uplink radio and mobile terminal battery resources more efficiently.